In 2011, digital books earned about $3.2 billion in revenue, an amount that the combined momentum of e-readers and tablets is expected to triple to $9.7 billion by the year 2016, according to a Juniper Research report.

Readers are showing increased loyalty to digital books, according to the US Book Industry Study Group (BISG).

Nearly half of print book buyers who also got digital works said they would skip getting an ink-and-paper release by a favorite author if an electronic version could be had within three months, a BISG survey showed.

"The e-book market is developing very fast, with consumer attitudes and behaviors changing over the course of months, rather than years," said BISG deputy executive director Angela Bole.

Concerns about e-book reading are diminishing, with people mainly wishing for lower device prices, according to the survey.

Owning e-readers tended to ramp up the amount of money people spent on titles in what BISG described as a promising sign for publishers.